Eddy-mediated biological productivity in the Bay of Bengal during fall and spring intermonsoons

The signature of cold-core eddies and their role in altering the biological productivity of the Bay of Bengal was examined using two recent sets of hydrographic data collected along the central and western Bay of Bengal during fall (14 September–12 October, 2002) and spring (12 April–7 May, 2003) intermonsoons under the Bay of Bengal process studies (BOBPS) programme. Based on the thermohaline structure and the satellite-derived sea-level anomaly maps nine cyclonic eddies were identified. Out of this, four cyclonic eddies—two each along the central Bay and along the western boundary—occurred during fall intermonsoon 2002, while five occurred—three along the central Bay and two along the western boundary—during spring intermonsoon. The eddy depressed the temperature, which varied from 3 °C to 7 °C at 120 m depth. Maximum depression of temperature was associated with spring-time eddies in the northern Bay, where subsurface stability was low. The reduced water column stability in spring leads to greater eddy-pumping, thereby cooling the water to a greater extent. However, the cyclonic eddies were unable to break the stratification of the top 20 m layer, thereby curtailing their effects below this depth during both seasons. Eddy-pumping not only cooled the water column but also enhanced the nutrient concentrations. This in turn increased the biological productivity of the Bay to 112-2 times. In addition, the subsurface chlorophyll maximum (SCM), which is generally located between 40 and 70 m in fall and 60 and 90 m in spring intermonsoons, shallowed under the influence of the eddies and also enhanced the chlorophyll concentration in the SCM to more than double. Thus, eddy-pumping of nutrients controls the biological productivity of the Bay of Bengal during both the seasons. In the fall intermonsoon, however, the riverine input of nutrients and sediments in the northern Bay also plays a role in altering the biological productivity. This has an overall implication to the basin-wide new production and export flux and, at least partly, resolves the reason for the comparable annual fluxes of organic carbon between the high-productivity Arabian Sea and low-productivity Bay of Bengal.